The present invention is directed to elimination of a temporary decrease in insulation resistance of polyester film capacitors.
During manufacturing of metallized poly(ethylene terephthalate) (PET) film capacitors, the dielectric is exposed at several stages to thermal and mechanical stresses. Examples of such fabrication steps are found in wound and stacked capacitor processes and include film metallization, flattening, heat treatment and curing of the resin.
An annoying side effect of these processing steps is the impact they can have on a key electrical property of the capacitor, the insulation resistance (IR). The IR of a capacitor is a measure of the leakage current at specified electrical field strength, charging time, temperature and relative humidity. Typically the IR is measured at short charging times of one minute or less which, at ambient conditions, is well before a steady-state conduction regime with time-independent currents is reached. Constant and reproducible resistivity values (10+.sup.18 ohm.multidot.cm) are reached for PET only after hours of continuous charging. IR is therefore not a measure of a fundamental material constant, but depends heavily on transient phenomena like dipolar relaxations or others. This transient nature makes the IR measurement inherently difficult.
Even if measurement conditions are carefully controlled, variations in the IR measurement by more than an order of magnitude are observed. They are attributed to manufacturing steps (a heat treatment for instance) in the capacitor process. Even though the IR normally recovers, it does so slowly and relaxation times are in excess of one week. This phenomenon can significantly influence IR quality control measurements. For instance, the nature of the heat treatment, the specific process and capacitor construction, the time elapsed between heat treatment and IR measurement all have their impact on the value recorded. Not only can these parameters be the cause for large variations in the IR measurement, they can also, in the right combination with other detrimental factors, cause the number of IR rejects to be unusually high.
Thus, a need is present for a technique to eliminate any decrease in insulation resistance in a capacitor.